This invention relates to droop stop mechanisms for use with fully articulated rotary wing aircraft and, more particularly, to such mechanisms that are virtually jam proof and substantially resistant to flapping induced damage.
When at rest or slowly rotating, helicopter blades droop downwardly to such an extent that, but for the provision of a mechanism stopping the droop, the tips of the blades would damage the fuselage and tail cone structures.
Rapidly rotating blades, however, are not subject to this danger. Whenever the blades are rapidly rotating, centrifugal and aerodynamic forces straighten the blades and eliminate the possibility of the tips being brought into damaging contact with the helicopter's fuselage. Accordingly, in the flight state, a comparatively larger degree of droop is permitted about the flapping axis of a fully articulated rotor than in the rest or slowly rotating (i.e., ground) state.
Droop stop mechanisms are designed to allow the larger degree of blade motion about the flapping axis in the flight state relative to that permitted in the ground state, as above described, and typically include a spindle pad having a flat bearing surface mounted to the root of blade connected structure; a vertical hinge having a flat bearing surface mounted to the rotor hub assembly and a pair of pendulum arms pivotally connected to the vertical hinge at a point intermediate its two ends. Each pendulum arm consists of a centrifugally responsive lower weighted end and an upper cam end having a flat bearing surface.
In operation, during the ground state, a spring and cooperating stop bias the flat bearing surface of the cam ends of the pendulum arms into upwardly supporting alignment with the flat bearing surface of the spindle pad such that a blade resting thereon is substantially horizontal. Damaging contact of the blade tips with the fuselage and tail cone structures is thereby effectively eliminated. During the flight state, a stop limits the clockwise rotation of the centrifugally responsive lower weighted ends of the pendulum arms such that the flat bearing surface of the vertical hinge acts as a dynamic droop stop upon which abuts the flat bearing surface of the spindle pad; this provides the larger degree of blade motion about the flapping axis permitted rapidly rotating blades. A droop stop mechanism of similar function is shown by U.S. Pat. No. 2,614,640 issued to Buivid.
However, whenever the pendulum arms are changing between the ground (flight) and the flight (ground) positions, the cam ends of the pendulum arms can jam if one of their corners is stuck by the flat bearing surface of the spindle pad. In such cases, as discussed more fully below in the detailed description of the invention, aerodynamically and inertially induced moments about the flapping axis may buildup to such an extent as to become potentially very damaging to the droop stop mechanism.
Accordingly, it is an object of this invention to provide a droop stop mechanism that is virtually jam proof and substantially resistant to flapping induced damage.
Another object is to provide a droop stop mechanism having full surface bearing contact for reacting blade moments over a wide range of cam angular rotation.
It is a related object to provide an improved droop stop mechanism which readily disengages whenever cam angular rotation is insufficient to provide adequate contact area for reacting blade moments.
An additional object is to provide a droop stop mechanism in which no moments are built-up about the pivot points of the pendulum arms.